Optical parametric oscillators are one class of laser apparatus that can provide continuously tunable laser output over a useful range of wavelengths. In an OPO an optically nonlinear crystal is arranged to divide the frequency of incident radiation into short and long wavelength portions. The incident radiation is typically referred to by practitioners of the art as pump radiation. The short and long wavelength portions of the frequency division are termed the signal and idler portions respectively. The sum of the signal frequency and the idler frequency is equal to the pump radiation frequency.
Typically it is the signal wavelength that is required for an application. The optically nonlinear crystal is located in an optical resonator arranged for circulating the signal wavelength radiation, providing optical parametric gain when pumped by the pump radiation. The idler radiation is allowed to escape the resonator. The signal and idler wavelengths can be tuned by varying the incidence angle of pump radiation in the optically nonlinear crystal or by varying the temperature of the optically nonlinear crystal.
Tunable CW laser-radiation in or near the red end of the visible spectrum is useful for applications in the biosciences. In order to provide signal radiation in that wavelength range, pump radiation must have a wavelength in the green or shorter wavelength region of the visible spectrum. Laser radiation at green or shorter wavelengths is typically generated by frequency-doubling near-infrared (NIR) radiation in or from a solid-state NIR laser such as neodymium-doped yttrium vanadate (Nd:YVO4) or neodymium-doped yttrium aluminum garnet (Nd:YAG) laser. These lasers have a fundamental wavelength of about 1064 nanometers. Frequency-doubling provides second harmonic (2H) at a (green) wavelength of about 532 nm. Intra-cavity frequency-doubled optically pumped semiconductor (OPS) NIR lasers have more wavelength flexibility, and can provide green laser-radiation in wavelengths at or around the 532 nm wavelength.
The optical parametric generation process is relatively inefficient compared with harmonic conversion due to some of the parametrically converted radiation being at an unwanted (usually the idler) wavelength. Frequency doubling to provide green radiation, particularly of CW radiation is also a low efficiency process. Accordingly the overall optical-to-optical efficiency of a current state-of-the-art, green-radiation pumped CW OPO could be as low as a few percent. An improved architecture for a CW OPO laser, providing improved conversion efficiency, could expand the application of CW OPO lasers as sources of tunable laser-radiation.